As disclosed in the van der Grinten Bulletin of June 22, 1967, one known form of Zig-Zag folding machine comprises folder means having first and second parts; first and second spaced folding rollers respectively co-operable with the first and second parts of the folder means to form first and second spaced folding devices; non-return feed means which prevent return movement of pliable strip material fed between the first and second spaced rollers; and movable guide means for directing the pliable strip material towards at least one of the spaced folding devices.
In this prior art folding machine, the first and second parts of the folder means respectively constitute third and fourth folding rollers and the folder means include an intermediate part extending between the third and fourth folding rollers and providing a fixed guide surface for the pliable strip material. The non-return feed means of this machine comprises a pair of feed rollers disposed upstream of the first and second folding rollers, at the entrance of the path of the pliable strip material between the first and second folding rollers. The movable guide means of the machine constitute an oscillatory guide extending from a fixed guide which extends downstream from the feed rollers and is movable between two positions in which the pliable strip material is directed, respectively, into the nip between the first and third folding rollers and into the nip between the second and fourth folding rollers.
In operation, the first and second folding rollers always rotate in the same direction as each other, first one way and then the other. The third and fourth rollers always rotate in the opposite direction to the first and second rollers. The first and second folding devices therefore alternate in rotating in such a way as to provide a folding nip for the pliable strip material from the feed rollers.
To start the folding operation, the oscillatory guide is pointed towards the first and third folding rollers which are rotated in such a way as to provide a folding nip and the leading end of the strip material is fed between the feed rollers, through the oscillatory guide and between the first and third rollers constituting the first folding device. All the folding rollers are then stop and rotated in the opposite direction and the oscillatory guide is swung away from its initial position until it is pointing towards the second and fourth folding rollers. As a result of the continuing operation of the feed rollers and the reverse rotation of the first and third folding rollers, a loop is formed in the pliable strip material and this loop is fed into the nip between the second and fourth folding rollers to form a first fold. All the folding rollers are then stop once again and then rotated in the first direction. Another loop is formed in the pliable strip material and this loop is fed into the nip between the first and third folding rollers to form a second fold. Third, fourth and subsequent folds may be formed in a completely analogous manner.
However, difficulty is encountered in providing means for synchronising movement of the oscillatory guide with the changes in direction of rotation of the folding rollers and with the rate at which slack is formed in the pliable sheet material so as to result in the controlled formation of loops which are fed alternately between the rollers of the first and second folding devices. It is therefore impossible to obtain completely accurate folding with adjacent folds lying on top of each other.
In a less complex, less costly, less bulky and less difficult to operate folding machine of similar construction, disclosed in U.S. Pat. No. 3,747,917, the first and second parts of the folder means comprise peripherally spaced portions of a single roller or spaced portions of an endless band trained around two parallel rollers. The non-return feed means of this machine do not feed the pliable strip material between the first and second rollers and into the nips provided by the first and second folding devices, but allow this movement and prevent movement of the pliable strip material in the opposite direction. Finally, the movable guide means of this machine are provided by an intermediate part of the folder means extending between the first and second parts. Thus, in one embodiment, the movable guide means are provided by a circumferential portion of the single roller and in another embodiment the movable guide means are provided by a part of the endless band providing the first and second parts of the folder means.
Thus, as the leading edge of the pliable strip material is fed between the first and second rollers and into engagement with the intermediate part of the folder means, movement of the folder means deflects the leading edge of the pliable strip material into which ever of the nips between the first and second folding devices towards which the intermediate parts is moving. In this way, as with the oscillatory guide, the first fold is formed at a predetermined distance from the leading edge of the pliable strip material.
Unfortunately, the intermediate part of the folder means does not provide positive guidance for the pliable strip material and, because the speed of the intermediate part of the folder means is limited to the speed at which the pliable strip material is fed into the first and second folding devices, there is unwanted slippage between the pliable strip material and the intermediate part of the folder means and this results in inaccurate folding.